In the document FR-A-2 568 640, an arrangement has been proposed which allows a significant displacement of the maximum possible angular extent to be effected between the second and third parts of such a torsion damping device, while at the same time preserving the mechanical strength of its various components.
In that arrangement, another damper plate, or auxiliary plate, is associated with the first damper plate and is parallel therewith, with, as between these two damper plates, an alternate arrangement of relative rotational displacement means whereby they are engaged with the hub. The effect of this arrangement is that, in a first circumferential direction of relative angular displacement between the first and second parts, one of the said damper plates initially operates by itself, while with the angular displacement in the opposite direciton, it is the other damper plate which initially operates by itself.
In the embodiment described, the circumferentially acting resilient means operative between the first and second parts are the same ones for both of the damper plates.
This arrangement has the disadvantage that, since the damper plates in question are necessarily offset axially, it is desirable, in order to balance out, by symmetry, the forces brought into play, that one of the damper plates be split into two auxiliary plates, which are then mounted on either side of the other damper plate, being connected to each other by axial spacers which pass through the main damper plate via openings formed for this purpose in the latter.
Besides an additional weakening of the mechanical strength of the damper plate, due to the need for these axial spacers to pass through it, such an arrangement also has two further consequences.
First of all, the total number of annular plates or wheels around the hub is increased, to the detriment of the axially extending bulk of the assembly, and at the cost of a significant complexity of the assembly, which is inevitably relatively expensive.
In addition, the two auxiliary plates which are thus provided must in practice constitute the two plates which are in the axial sense the outermost plates of the assembly. This means that standardisation to an arrangement in which it would by contrast be the plates of the third part (one of which carries the friction disc where the assembly is a friction clutch for an automotive vehicle) which are axially the outermost ones, cannot be adopted.